Science a GoGo's Home Page Forums General Discussion Physics Forum Does Time Exist

I'm not going to make any argument one way or the other. I think it does, but what do I know? Anyway, just for Bill S., here is a link to Sabine Hossenfelder's latest.

Dear Dr. B: What do physicists mean when they say time doesn’t exist?

Bill Gill

That is actually a good link for Bill S, thank you Bill. It is actually covering part of the issue we were discussing elsewhere. I hadn't considered coming that way into the problem but it will get him there. I am sort of regretting going into this with him at the moment I think it would have been easier to say I agree with him AKA lie.

You are a classical man Bill, time is always real for you. Stick with that and it won't get into any trouble and most times you will get the right answer. This stuff only becomes problematic when you try to take a zero frame to the universe and you open Pandora's box.

Sabine's answer is reasonable, however I have to nit pick over the reversal problem she said quote "you would have to arrange the molecules in the dough so precisely that it’s impossible to do". Actually uncertainty principle says it is completely and utterly impossible you can not determine the exact position of a single molecule much less all the ones in the mixture. Idea was correct execution left open some idea that it might be insanely hard but possible. Then she went and did it, said it might be possible to unmix a small patch of dough based on an experiment which has a different thrust, and why she does my head in at times. It's clear she doesn't get the experiment she linked to is about energy at small scales and the mixed dough is about positional accuracy these sort of details are important. So again I am left in the situation with Sabine that she gets enough right to read her articles but I can never implicitly trust her articles.

With the energy experiment she highlights what surprised me was anyone needed to actually do it or that it was newsworthy. At times you wonder at what point will some just accept that classical physics isn't coming back and almost all it's laws require some handwaving.

Thanks for the link, Bill. No time to read it at the moment, but I'm printing it so I can take it with me.

It's soap-box time!! I have to nit pick the nit pick, and ask:

What's the difference between "impossible" and "completely and utterly impossible"?

From the experts' point of view, that might be self evident, but for the benefit of hitch-hikers every expert who writes on this subject should include this. I would even forgive them for writing it in upper case.

Well lets see Sabine said it's impossible then she goes on


So perhaps you need to take the issue up with HER. The impossible part is a correct statement the adjustment after is complete garbage and so I guess I have to put "completely and utterly" because obviously impossible is not absolute with her.

As I said she does my head in she gets some very complicate things correct but then completely botches basic things any undergrad would get right. It's like she read or got told the impossible bit but it never sunk in they really really mean it. The garbage after is all her you won't find it in anywhere else.

If you are asking me what she meant with that I have no idea it looks like some sort of idea out of chemistry that is completely butchered.

Again for QM interactions many things are simply not directly reversible not even conceptually they are only reversible via a different mechanism (Antiunitary and antilinear transformations).

I don't like her example with the dough, it's too easily misinterpreted, but consider the box of gas example. The box is divided by a partition with a small hole through which molecules can (and do) pass. If there are just two molecules in the box, the equilibrium state would be one molecule in each side. Would it not be possible, occasionally, to find both molecules in one side?

Let’s go further into your field. Our divided box (no need for a hole, now) has one quon in each side. Doesn’t uncertainty say we could find both in one side in any particular observation?

In both situations there is a probability that you will find them both sides and so entropy is unchanged it just becomes probability based on size of hole versus size of particle etc. This problem was realized in the 1870's see the difference between Classical thermodynamics and Statistical mechanics (https://en.wikipedia.org/wiki/Entropy).


So using Statistical mechanics in your example the entropy has not changed. So another one of those examples where you need to throw the classical description out.

You are however going along the right lines now what you do is introduce a little demon who controls the gap. The problem is called Maxwell's Demon (https://en.wikipedia.org/wiki/Maxwell's_demon) and it was argued it theoretically breaks the 2nd law.

When attempts to do the experiment with QM the problem became apparent. It costs you energy to measure where the particle is and to effect the control of the gap and that energy is always more than the energy of the entropy change.

This was the result with a single electron maxwell demon
http://arstechnica.com/science/2014/09/researchers-create-a-maxwells-demon-with-a-single-electron/

You may be curious to see Sabine had to learn the same lesson the hard way:
http://backreaction.blogspot.com.au/2014/12/the-remote-maxwell-demon.html
Give her half marks she did first worked out she needed to change to statistical mechanics but minus half the marks for not working out fully why her machine fails correctly.

In 2012 another group of Japanese physicists claimed they could do a maxwell's demon using entanglement
link: http://www.technologyreview.com/view/428...say-physicists/
It will fail for the same reasons as all the others and is no better than Sabine's.

Strangely the first correct argument of the impossibility of Maxwell's Demon was first made in 1961 by IBM founder Rolf Landauer in Landauer's principle (https://en.wikipedia.org/wiki/Landauer's_principle). You will note in the discussion it was widely accepted in QM and then experimentally verified.

It costs a small amount of energy to encode QM information and as no gain mechanism is known QM can not create Maxwell's demon.

You see with the stupid papers above why it is important that you learn the QM foundations. Those who do not bother find they may end up looking like idiots.

Isn't the entropy lower if the gas molecules are both on one side, than if they are one on each side?

If the targeted atom loses more energy than is put in; wouldn’t the system continually lose energy, and increase its entropy?

Only in classical physics you haven't confined the particles so under statistically mechanics nothing has changed .. did you read the entropy link at all?

Lets turn this to an electron orbital around an atom. Classical physics would likewise say the entropy and energy changes depending where the electron in it's shell relative to the nucleus and so with the probability function of QM both must change right? Only problem is experimentally that isn't what you measure you measure just one exact value which I imagine you know.

This is that problem again that the universe seems to have at it's core probability. If you use the statistical probability you will derive the correct entropy value and any calculation using an implicit time or position of the system will be wrong which is what classical physics and you did.

It gets far worse and far deeper when you consider transmitted messages in your above example you have a physical system but what if I implemented it by transmitting messages. You may care to read the section on information theory under entropy.

You live in a Quantum Universe and the correct description goes, EVERYTHING NOT FORBIDDEN IS MANDATORY. It is called the totalitarian principle (https://en.wikipedia.org/wiki/Totalitarian_principle).

A particle moving from A to B takes every possible path from A to B simultaneously. Two particles A and B in your container are actually simultaneously everywhere in your container with a probability density and distribution to find them, that is why the entropy doesn't change.

There is also a small non zero probability you will find them outside the containers, yeah that problem again

Until you close the gap between the sides the particle probabilities are unchanged regardless of where you might measure them.

The moment you made the two particles classical and tried to calculate an entropy of a single position you were doomed to get the wrong answer. Again the warning be very careful with classical physics. The easy way to stop making the error is stop making particles real.

Again this is Sabine I struggle to know what she is thinking at times as she seems to confuse herself.

I get the impression she thinks you wait for a random QM fluctuation to excite an atom (which cost you nothing) and then hit that excited atom getting twice the energy. I think she then views the atom drops back to repeat the process so it's not losing energy like you suggest but stealing it from QM or the universe ... she has a perpetual machine extracting energy with no change in the universe system save the temporary excite state.

Unfortunately she keeps ignoring the "if you knew" bit would require a measurement and that costs more energy and increases entropy more than the energy she would get back.

Orac, this just won't do. We seem to be on the same side.

DO SOMETHING!

The original question was: “Does time exist”.

Taking considerations from other threads, I find myself wondering if the answer might be:

Yes, it exists in the same way that “Pi”, “i” and “e” exist. It’s there, as long as there is something to measure with it.

Now you have got very close to what QM says but it extends it a tiny bit, that time exist as long as a field exists. It is after all a field that you are measuring and you need a field to do any measurement. So the interesting question it poses is does time exist in a place with no fields and the answer is you will never know because you can't measure. That is why in the big bang assumes time begins there, at least for us as Quantum beings, because we have the appearance of the first quantum field.

I have no problem with that, and if that is as far as you need to go, that's fine.

Kurt Godel had some interesting (weird?) thoughts about time which seem relevant to this thread. The following is an extract from John L. Bell, "Time and Causation in Gödel’s Universe".

In 1949 the great logician Kurt Gödel constructed the first mathematical models of the universe in which travel into the past is, in theory at least, possible. Within the framework of Einstein’s general theory of relativity Gödel produced cosmological solutions to Einstein’s field equations which contain closed time-like curves, that is, curves in spacetime which, despite being closed, still represent possible paths of bodies. An object moving along such a path would travel back into its own past, to the very moment at which it “began” the journey. More generally, Gödel showed that, in his “universe”, for any two points P and Q on a body’s track through spacetime (its world line), such that P temporally precedes Q, there is a timelike curve linking P and Q on which Q temporally precedes P. This means that, in principle at least, one could board a “time machine” and travel to any point of the past. Gödel inferred, in consonance (as he observes) with the views of Parmenides, Kant and the modern idealists, that under these circumstances there could be no such thing as an objective lapse of time, that time or, more generally, change, is an illusion arising from our special mode of perception. For consider an observer initially at point P (with time coordinate t seconds as indicated by his own clock). At point Q (with time coordinate t';) he boards a time machine and travels back to point P, taking time t'' to do so. In that case, according to his own clock, t'– t + t'' > 0 seconds have elapsed, and yet an identical clock left at
P would show that 0 seconds have elapsed. In short, there has been no “objective” lapse of time at all. Gödel remarks that in his universe this situation is typical: for every possible definition of an “objective” time one could travel into regions which are past according to that definition. He continues:
“This again shows that to assume an objective lapse of time would lose every justification in these worlds. For, in whatever way one may assume time to be lapsing, there will always exist possible observers to whose experienced lapse of time no objective lapse corresponds... But if the experience of the lapse of time can exist without an objective lapse of time, no reason can be given why an objective lapse of time should be assumed at all.”

Some comments on this could make interesting reading.

To be honest this is well covered in wikipedia and Tim did really well on that section

Godel Metric

Key points I would highlight for you

- It isn't a viable model for the universe but it is an exact solution to Einsteins equations, so this universe in theory could exist

- It highlights the problem of observers disagreeing and finding a good reference frame in the model is challenging.

- If you follow Hawking's logic the model shows you what a rotating universe would look like and we can rule out our universe is rotating.